1. Field of the Invention
The present invention relates to a substrate cleaning method and a substrate cleaning apparatus in which a chemical solution is used to clean and etch a substrate such as a semiconductor wafer, an LCD glass substrate, or the like.
2. Description of the Related Art
For example, in a wafer processing step of an LSI manufacture process, a substrate cleaning apparatus of a wet type is used to remove foreign material such as particles, organic material, metal impurities, and the like. In this wet type substrate cleaning apparatus, a chemical solution such as a hydrofluoric acid solution, an ammonia hydrogen peroxide solution, such as an aqueous solution of sulfuric acid and hydrogen peroxide; an aqueous solution of hydrochloric acid and hydrogen peroxide; a phosphate solution; a solution of hydrofluoric acid and nitric acid; an aqueous solution of hydrofluoric acid and hydrogen peroxide; and a solution of hydrofluoric acid and ammonium fluoride or the like is circularly supplied to a cleaning vessel, and a number of wafers are dipped in the chemical solution.
In the wafer processing step, circuits are patterned only on the front surfaces Wa of wafers. Therefore, in general the surface Wa of each wafer is mirror-finished by a super precise polishing method, while the back surface of each wafer is finished more roughly than the front surface Wa.
As shown in FIG. 20, wafers W are contained in a cassette C at equal pitch intervals such that the front surfaces Wa of the wafers are all oriented to the same side (in the grip side of the cassette). In the cassette, the front surface Wa of the wafers respectively face the back surfaces Wb of adjacent wafers. This arrangement of wafers is common to both the cassette C and the cleaning vessel.
Back surfaces of the wafers Wb are, in many phases of the manufacturing process, in contact with supporting members, carrier members, and the likes. Therefore, foreign material such as particles tend to more easily stick to the back surfaces Wb of the wafers than to the front surfaces Wa. It is generally said that a greater amount of foreign material sticks to the back surface Wb of a wafer than to the front surface Wa thereof.
Cleaning processing of wafers is carried out in a manner in which an upward flow is generated in a cleaning vessel by supplying a chemical solution to a cleaning vessel through a bottom portion so as to overflow to an upper portion of the vessel, and a plurality of wafers are dipped in the upward flow of the chemical solution.
However, in case of a group of wafers arrayed as shown in FIG. 20, foreign material which once has left the back surfaces Wb of the wafers tends to easily stick again to the front surfaces Wa of adjacent wafers. In particular, in case of cleaning wafers having a large diameter, foreign material easily sticks again to the front surfaces Wa of the wafers so that it is difficult to increase the cleaning efficiency of the wafers.
In addition, if wafers W.sub.L1 and W.sub.L2 are arrayed on a board 91 at a narrow pitch interval as shown in FIG. 15, these wafers easily incline so that upper portions of adjacent wafers W.sub.L1 and W.sub.L2 are easily brought into contact with each other since a clearance exists between a groove 98 and the wafer W.sub.L1 and between a groove 99 and the wafer W.sub.L2. In particular, when lifting up the board 91 from the cleaning vessel, a chemical solution is kept between the front surface Wa of the wafer W.sub.L1 and the back surface Wb of the wafer W.sub.L2, resulting in that non-uniformity in the cleaning rate appears within the surface of a wafer and that the cleaning rate varies between wafers arrayed in the same lot. However, if wafers W.sub.L1 and W.sub.L2 of a large diameter are arrayed at a wide pitch distance, the size of a cleaning vessel must be enlarged and a long time is required for circularly supplying a chemical solution into the cleaning vessel, so that the through-put is lowered.
Furthermore, the chemical solution is required in a large amount, increasing treatment in cleaning cost. Moreover, it takes a long time to heat the chemical solution to a predetermined temperature.